1. Field of the Invention
The present invention relates to a surface acoustic wave device and a communication apparatus, and more particularly concerns a surface acoustic wave device and communication apparatus especially preferable for use in mobile radio communication and cellular radio communication, such as mobile telephones and pagers
2. Description of the Related Art
A conventional surface acoustic wave filter is formed on a piezoelectric substrate by combining a transducer for converting an electrical signal to surface acoustic wave and a transducer for converting the surface acoustic wave to electric signal. Surface acoustic wave filter of this type is described, for example, in an article in the IEE Trans., "Microwave Theory and Techniques", MTT-22, (1985) pp. 510-518. In a prior mobile telephone system for simultaneous transmission and reception with use of a single antenna, the antenna is connected to the input or output of a transmitter surface acoustic wave filter and receiver surface acoustic wave filter through an appropriate feeder line. It is described, for example, in an article in the IEEE Trans., "Microwave Theory and Techniques", MTT-36, (1988) pp. 1047-1056. The transmitter surface acoustic filter and receiver surface wave filter are implemented in a single module, and an antenna duplexer is connected to an antenna terminal of a two-way receiver of the mobile telephone system.
A prior filter is constructed with coupled SAW resonators is described, for example, in the Japanese Patent Laid-Open 63132515 and consists of a combination of one-port surface acoustic wave resonators formed on a single piezoelectric substrate. The filter characteristic synthesis or cutoff frequency response is determined by electrode structure of the resonator.